1. Field of the Invention
The present invention relates to a control technique for internal combustion engines, and more particularly, to a control technique for preventing an engine speed from abruptly increasing to an excessively high speed immediately after the start of the engine.
2. Background Art
Immediately after the start of an internal combustion engine, the engine speed abruptly increases well above an idling speed. This is due to the fact that whereas the internal pressure of the air intake pipe under normal operating conditions including an idling state is a negative pressure, the internal pressure of the air intake pipe during engine start is an atmospheric pressure. During start of the engine, as in an idling state, a throttle valve is closed and an ISC valve is opened to a required idling angle position. Since the air density inside the air intake pipe during the start increases above the air density reached under normal operating conditions, the amount of air taken into a combustion chamber immediately after the engine has been started up increases above the amount of air taken in during idling, even for the same valve angle. Consequently, the output of the engine increases according to the particular increment of the air intake rate, thus causing the engine speed to increase above the idling speed.
The technique described in Japanese Patent Laid-open No. 2001-304084 is known as a conventional technique for preventing an engine speed from abruptly increasing to an excessively high speed. This technique suppresses an increase in engine speed by retarding ignition timing with respect to idling ignition timing, for a required time after the start of the engine, and reducing the engine output.
However, it is difficult to preset the time for retarding the ignition timing, since the rotational behavior of the engine after it has been started changes under the influence of air density, an air/fuel ratio (volatility of the fuel), friction (oil viscosity and a rod-cylinder fit), and other factors. Trouble is likely to arise, depending on the particular relationship between the retarding time of the ignition timing and the rotational behavior after the engine start. For example, if the fuel is heavy in terms of properties and the rate of the abrupt increase of the engine speed is lower than usual, the engine speed is likely to suddenly decrease, thus causing the engine to stall. Conversely, if the fuel is light in terms of properties or friction is low and the rate of the abrupt increase of the engine speed is higher than usual, the abrupt increase of the engine speed may not be sufficiently preventable.